The present invention relates to the field of carboxylic esters and its subject is more particularly their synthesis by direct reaction of a carboxylic acid and an alcohol.
This reaction is generally catalyzed by an acid which may be sulphuric acid or an acidic resin, but very frequently a sulphonic acid Rxe2x80x94SO3H such as methanesulphonic acid because sulphonic acids make it possible, among other advantages, to obtain carboxylic esters of better quality. However, sulphonic acids, and in particular methanesulphonic acid, have the disadvantage of forming, during the reaction, a small quantity of sulphonic esters of the alcohol which bring about the presence of residual sulphur in the desired carboxylic ester and whose hydrolysis generates the formation of free acidity which is harmful to the quality of the product.
In the case of light carboxylic esters which can be distilled without degradation, the problem may be solved by using a heavy sulphonic acid for which the ester formed may be separated from the carboxylic ester by distillation. However, this solution is not appropriate in the case of degradable carboxylic esters; the reaction product must in this case be treated separately on a resin in order to remove the sulphonic ester formed.
It is known that an irreversible quaternization reaction occurs between a tertiary amine NR1R2R3 and an alkyl-donating compound such as a sulphonic ester Rxe2x80x94SO3-Alkyl according to the following scheme:
Rxe2x80x94SO3-Alkyl+NR1R2R3xe2x86x92R1R2R3AlkylN+ Rxe2x80x94SO3xe2x88x92
This reaction has even been used to carry out post-treatments of esters so as to remove therefrom certain sulphonic esters which are present. However, this requires an additional step in the process for the manufacture of the esters and is not therefore easy to carry out.
It has now been found that this quaternization reaction can occur during the reaction for the synthesis of the carboxylic esters itself. Not only is the quaternization reaction possible and makes it possible to remove the sulphonic esters as they are formed, but in addition the presence of the tertiary amine does not at all modify the performance of the sulphonic acid catalyst.
The subject of the invention is therefore a process for the preparation of a carboxylic ester by direct reaction of a carboxylic acid and an alcohol catalyzed by a sulphonic acid R-SO3H, characterized in that the procedure is carried out in the presence of a solid comprising tertiary amine functions in free form.
As solids comprising tertiary amine functions, there may be used in accordance with the present invention organic polymers or copolymers with tertiary amine functions which are well known in the art as anion exchangers. As such, it is possible more particularly to use polystyrene-based resins which are crosslinked in particular with divinylbenzene, acrylic or phenylacrylic resins, acrylic resins which are crosslinked with divinylbenzene, or resins of the phenol-formaldehyde type. These resins carry tertiary amine functional groups which are attached by various techniques known per se, generally after formation of polymers or copolymers. There may also be mentioned the polyvinylpyridines obtained by polymerization of 4-vinylpyridine. Such anion-exchange resins are commercially available under various names such as, for example, Amberlite, Duolite, IRA and Reillex.
The quantity of solid to be used can vary within wide limits. It is generally between 0.01 and 20% relative to the weight of the reaction mixture (acid+alcohol+optional solvent), preferably between 0.1 and 5%.
The process according to the invention may be applied to the preparation of any carboxylic ester whose direct synthesis from the corresponding carboxylic acid and alcohol is catalyzed by a sulphonic acid Rxe2x80x94SO3H. It applies in particular to the synthesis of the esters derived from carboxylic acids such as acrylic or methacrylic acids or fatty acids such as heptanoic acid or stearic acid, and mono-, di- or polyfunctional alcohols such as isopropanol, 2-ethylhexanol, diisopropylene glycol, diisobutylene glycol, polyethylene glycols or trimethylolpropane.
As sulphonic acids R-SO3H, there may be mentioned more particularly those in which R is an aliphatic or aromatic radical, such as the acids CH3(CH2)nSO3H with n ranging from 0 to 3 and p-toluenesulphonic acid. The preferred sulphonic acid is methanesulphonic acid.
The esterification reaction proper is carried out under the usual operating conditions, that is to say:
carboxylic acid/alcohol molar ratio ranging from 1 to 10, preferably of between 1 and 5,
temperature of between 50 and 200xc2x0 C., preferably of between 80 and 150xc2x0 C.,
quantity of sulphonic acid Rxe2x80x94SO3H of between 0.1 and 5% relative to the weight of carboxylic acid, preferably of between 0.5 and 2%.
It is possible to carry out the procedure in the absence of solvent or in a mixture with a solvent such as toluene or heptane. The quantity of solvent may vary within wide limits, but it is generally between 5 and 95% relative to the weight of the reaction mixture (acid+alcohol+solvent), preferably between 5 and 50%.
The esterification reaction may be carried out batchwise, semicontinuously or continuously.
At the end of the reaction, the solid may be separated by simple filtration or any other equivalent means.